OMG hope he didn't leave a cat,gerbil,hamster,mouse,ferret, or other
Enduring smells like that are often caused by something dead. Mouse, rat, whatever and it doesn't mean the PO left it, it could have snuck in. It's hard to imagine what else would for no apparent reason start to smell.
On a previous boat we had a small bird crawl into the air intake port into the engine room and got stuck and died. It was easy to find and didn't smell for long but a dead rat or similar will resist just about any chemical treatment and might even be made worse by chemical exposure. They smell bad anyway and are quite durable. If it is something like that and you can't reach it, the smell will eventually go away but it's the thought of the remains in the boat that are a little unpalatable.
Are there no other signs of "visitors"?